About 90% of the total butane consumption in the United States is in gasoline manufacture where n-butane is used directly as a blending component, and isobutane is either used for the production of high octane alkylate or for the production of isobutylene to make tert-butyl ether. Chemical uses account for another 6-8% of the total butanes. Due to the recent increased demand for high octane gasoline and the federally regulated reduction of gasoline vapor pressure, there is the need to have a process that can effectively convert normal butane to the higher octane rated isobutane, to ultimately increase the production of light octane blending components.
Current butane isomerization processes, using either aluminum chloride or chlorinated platinum on alumina or zeolite catalysts, require high operating temperature and use hydrogen as a cracking suppresser. Thus, the commercial processes could only operate in the vapor phase. We have discovered a liquid phase process for the isomerization of normal butane to isobutane. The liquid phase operation provides engineering design and operating cost advantages over conventional vapor phase processes. In the process design area, savings are available due to reduced size of liquid containing lines, reduction of pump energy requirements and elimination of the compressors needed in a vapor phase process to overcome pressure drop through the process, and use of lower cost equipment for heat input by avoiding vaporization of the butane feed. These equipment simplifications also result in less utility requirement for the drivers for pumps and compressors, and eliminate the need of fired heat for vaporization and of cooling water for condensing. In addition, the liquid phase operation also provides other benefits such as lower catalyst deactivation rates than those in the vapor phase operations.
Pentane isomerization is also of commercial interest and is presently carried out with similar platinum on zeolite catalyst and in the vapor phase. The design and cost advantages for a liquid phase pentane isomerization are similar to those described above for butane liquid phase isomerization. C.sub.6 and C.sub.7 alkanes may also be isomerized according to the invention.
The possibility of liquid phase alkane isomerization is enhanced by the recent discovery of a very active solid superacid catalyst system which is disclosed and claimed in pending application of Hollstein, Wei and Hsu, Ser. No. 247,225 filed Sep. 21, 1988, now U.S. Pat. No. 4,918,041. However the invention is applicable to superacid catalysts generally which have activity for alkane isomerization.